Conventional crystalline silicon photovoltaic cells, such as solar cells, are generally made of thin wafers of silicon (Si) in which a rectifying or p-n junction has been created and electrode contacts, that are electrically conductive, have been subsequently formed on both sides of the wafer. A solar cell structure with a p-type silicon substrate has a positive electrode contact on the base or backside and a negative electrode contact on the n-type silicon or emitter that is the front-side or sun-illuminated side of the cell. The “emitter” is a layer of silicon that is doped in order to create the rectifying or p-n junction and is thin in comparison to the p-type silicon substrate. Radiation of an appropriate wavelength falling on a p-n junction of the silicon semiconductor substrate serves as a source of external energy to generate hole-electron pairs in the semiconductor substrate. Due to the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions. The electrons move to the negative electrode contact, and the holes move to positive electrode contact, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit.
Most crystalline silicon solar cells are fabricated with an anti-reflection insulating layer comprising silicon nitride, silicon dioxide, titanium dioxide or other materials with a similar refractive index. This layer acts as the anti-reflection coating (ARC) on the front-side of the cell to maximize sunlight absorption. Front-side electrode contacts are generally made by screen printing conductive fingers and buss bars using a thick-film silver conductive paste on the anti-reflection coating followed by firing at an elevated temperature. Conventional front electrode thick-film silver pastes contain silver powder, an organic medium, a solvent, a glass frit and may contain various additives. The silver powder functions as the main electrode contact material and provides for low resistance. The glass frit may contain lead or other low melting point constituants to give a softening point of about 300 to 600° C. so that during firing, the glass frit becomes molten and functions as the “fire through” or “etching” agent wherein the silicon nitride anti-reflection coating is penetrated by the paste to allow the silver to make electrical contact to the n-type silicon. The glass frit also provides for adhesion of the silver to the silicon. Compositions and firing profiles of the thick-film silver conductive paste are optimized to maximize cell efficiency.
FIG. 1 is a process flow diagram, shown in partial cross section, illustrating the fabrication of a photovoltaic device according to conventional processes and with conventional materials.
In FIG. 1A, a p-type silicon substrate 10 is provided. The substrate may be composed of single-crystal silicon or of multicrystalline silicon. As shown in FIG. 1B, in the case of a p-type substrate, an n-type silicon layer 20 is formed to create a p-n junction. The n-type layer is typically formed by the thermal diffusion of a donor dopant, preferably phosphorus (P) using phosphorus oxychloride (POCl3). The depth of the diffusion layer is generally about 0.3 to 0.5 micrometers (μm). The phosphorus doping causes the surface resistance of the silicon to be reduced to between several tens of ohms per square (Ω/□) to something less than 100 ohms per square (Ω/□). In the absence of any particular modification, the diffusion layer 20 is formed over the entire surface of the silicon substrate 10.
Next, one surface of the diffusion layer 20 is protected with a resist or the like (not shown) and the diffusion layer is removed from all but one surface of the article of FIG. 1B by etching. The resist is removed, leaving the article of FIG. 1C.
Next, as shown in FIG. 1D, an insulating layer of silicon nitride SiNx:H film is formed on the above-described n-type diffusion layer to form an anti-reflection coating (ARC). The thickness of the SiNx:H anti-reflection coating 30 is about 700 to 900 Å. As an alternative to silicon nitride, silicon oxide or titanium oxide may be used as the ARC.
As shown in FIG. 1E, a silver paste 50 for the front electrode is screen printed and then dried over the silicon nitride ARC 30. In FIG. 1E only two front electrode fingers are shown for purposes of clarity and simplicity. In practice, many are printed. In addition, an aluminum paste 60 and a backside silver or silver/aluminum paste 70 are screen printed and successively dried on the backside of the substrate. Co-firing of front and backside pastes is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. in air for a period of from several seconds to several tens of minutes.
As shown in FIG. 1F, aluminum diffuses from the aluminum paste into the silicon substrate 10 as a dopant during firing, forming a p+ layer 40 containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the photovoltaic cell.
Firing also converts the aluminum paste 60 to an aluminum back electrode 61. The backside silver or silver/aluminum paste 70 is fired at the same time, becoming a silver or silver/aluminum back electrode 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, thereby achieving electrical connection. The aluminum electrode accounts for most of the back, owing in part to the need to form a p+ layer 40. Because soldering to an aluminum electrode is impossible, a silver back tab electrode is formed over portions of the back side as an electrode for interconnecting photovoltaic cells by means of a copper ribbon or the like.
During the co-firing, the front electrode-forming silver paste 50 sinters and penetrates through the ARC 30, and is thereby able to electrically contact the n-type layer 20. This type of process is generally called “fire through” or “etching” of the silicon nitride ARC. This fired through state is apparent in layer 51 of FIG. 1F.
FIG. 2 schematically illustrates a conventional front electrode design for a silicon solar cell 200 in plan view and as partially shown in cross section in FIG. 1F. The solar cell anti-reflection coating is shown as element 230 in FIG. 2 and is represented as element 30 in FIG. 1F. Numerous silver-containing fingers 251 of the front electrode design are arranged over the area of the solar cell. Fingers 251 are represented by the element 51 in FIG. 1F. In FIG. 2, only 28 fingers 251 are shown for clarity. However, the number of fingers may be much higher in a typical 6 inch by 6 inch crystalline silicon solar cell. The number of fingers 251 are designed to maximize the extraction of electric current from the solar cell and depends on the width of the fingers. A typical width of fingers 251 may be 100 micrometers and a typical number of fingers 251 on a 6 inch by 6 inch solar cell may be 60. Other widths and numbers of fingers 251 are feasible. Buss bars 280 contact the fingers 251 and are arranged perpendicular to the fingers 251. The buss bars serve to conduct and carry current from the fingers 251 to external circuitry. Designs with two buss bars are common but other designs are feasible. The width of a typical buss bar is approximately 2 mm. In conventional cell fabrication, fingers 251 and buss bars 280 are printed at the same time with silver thick-film paste. Using the same type of thick-film silver paste for the fingers and buss bars has allowed for the firing of the conductive fingers and bus bars in a single firing with a single set of firing conditions such as temperature and firing atmosphere. The use of the same or similar thick-film silver paste for the fingers and the buss bars has been considered necessary to minimize the number of firings and to minimize cell damage associated with multiple high temperature firings. A larger number of firings is more likely to reduce the efficiency of the solar cell.
There is a need for greater flexibility in the conductive pastes used for forming the buss bars for the sun-facing side of solar cells. High silver costs and limited supply of silver make it important to be able to form buss bars for the sun-facing side of solar cells from less expensive and more available conductors. Novel compositions and processes for forming front side buss bars of photovoltaic devices are needed, which make possible the reduced use of silver pastes.